1. Field of the Invention
The present invention relates to stencil printing machines and more particularly to a stencil printing machine equipped with plural printing drums for achieving a multi-color printing and a method of controlling the same.
2. Description of the Related Art
Various research and development work has been undertaken to provide an improved stencil printing, a typical example of which is disclosed in FIG. 6 which shows a part of a structure of the earlier stencil printing machine. In FIG. 6, first and second printing drums 101 and 102 and a press drum 103 are rotatably supported in a frame body (not shown) such that the first and second printing drums 101 and 102 are located in close proximity to an outer periphery of the press drum 103 at positions angled at 90 degrees of a central angle of the press drum 103. Outer circumferential peripheries of the respective first and second printing drums 101 and 102 carry thereon stencil clamping bases 101a and 102a, respectively, which support thereon sheet clamping segments 101b and 102b for clamping stencil sheets (not shown) onto the stencil clamping bases 101a and 102a, respectively.
Further, screens 105 are wound on the outer circumferential peripheries of the first and second printing drums 101 and 102 in a stretched state with the use of the stencil clamping bases 101a and 102a and springs 104, with each of the screens 105 being formed of a mesh structure which allows printing ink to permeate. Inner press rollers 106 and 107, which serve as ink supply rollers, respectively, are located inside the screens 105 of the first and second printing drums 101 and 102, respectively, with the inner press rollers 106 and 107 being moveable between a wait position not to press the screens 105 and a press engagement position to press the screens 105.
During printing operation, the inner press rollers 106 and 107 are maintained in the press engagement position, in which the screens 105 are expanded outward. Also, it is arranged such that the screen 105 of the first printing drum 101 is supplied with printing ink in a first color by the inner press roller 106 and the screen 105 of the second printing drum 102 is supplied with printing ink in a second color by the inner press roller 107. The outer circumferential periphery of the press drum 103 is provided with a print sheet clamping segment 109, for clamping a leading edge of a print sheet 108, which clamps the leading edge of the print sheet 108 transferred from a paper feed section 110 and release the leading edge of the print sheet 108 at a position in the vicinity of an inlet portion of a sheet discharge section 111.
Now, the stencil printing machine thus arranged operates as follows. A leading edge of a first stencil sheet, which has been made on the basis of image data in a first color of an original is clamped with the sheet clamping segment 101b of the first printing drum 101, and a leading edge of a second stencil sheet, which has been made on the basis of image data in a second color of the original, is clamped with the sheet clamping segment 102b of the second printing drum 102, with the stencil sheets being mounted onto the outer circumferential peripheries of the respective screens 105. Next, the first and second printing drums 101 and 102 and the press drum 103 are rotated in synchronism with one another in directions as shown by arrows in FIG. 6, thereby causing the print sheet 108 to be transferred between the first printing drum 101 and the press drum 103 from the paper feed section 110. The print sheet 108 thus transferred is clamped with the print sheet clamping segment 109 of the press drum 103, allowing the print sheet to pass along the outer circumferential periphery of the press drum 103 between the first printing drum 101 and the press drum 103.
During this passing step of the print sheet, the inner press roller 106 is brought into press engagement with the screen 105 of the first printing drum 101 which is consequently expanded outward, allowing printing ink to be transferred to the print sheet 108 to reproduce a desired image pattern with a first color via a perforated image area of the first stencil sheet. The print sheet 108, which has passed between the first printing drum 101 and the press drum 103, then passes between the second printing drum 102 and the press drum 103. During this passing step of the print sheet, the inner press roller 107 is brought into press engagement with the second screen 105 which is consequently expanded outward, allowing printing ink to be transferred to the print sheet 108 to reproduce a desired image pattern with a second color. As the sheet clamping segment 109 of the press drum 103 is rotated to a position near the inlet of the sheet discharge section 111, the sheet clamping segment 109 is released, with the released print sheet 108 being discharged to the given discharge position by the sheet discharge section 111. In this manner, two-color printing is completed.
As noted above, the first and second printing drums 101 and 102 are located at the rotational positions spaced by 90 degrees of central angle of the press drum 103, in which the first and second printing drums 101 and 102 are rotated with the rotational phase angle of 180 degrees such that the respective printing positions of the first and second printing drums 101 and 102 are aligned with one another relative to the press drum 103. As a consequence, the stationary rest positions of the respective printing drums 101 and 102 are kept in opposing positions of 180 degrees at the termination of the printing operation.
By the way, in the earlier practices, since no consideration has been undertaken to the rotational stop positions of the respective printing drums 101 and 102 at the termination of the printing operation, when, for example, the first printing drum 101, which is one of the printing drums, is stopped at the stationary rest position in an upper area, the second printing drum 102, which is the other remaining printing drum, is caused to stop at the stationary rest position located in a lower area, with this stationary rest position being left until the start of next printing operation.
On the other hand, in the event ink pools are located at upper circumferential positions of the respective inner press rollers 106 and 107 and supply printing ink to the inner press rollers 106 and 107, the printing ink is liable to gradually flow downward in a long time period along the respective outer peripheries of the inner press rollers 106 and 107. Consequently, when the stencil clamping bases 101a and 102a remain at the lower positions, the dropped printing ink.flows through a gap between the each screen 105 and each stencil clamping base 101a or 102a onto a surface of each stencil clamping base 101a or 102a, causing that surface to be stained with the printing ink.
The present invention has been made to address various issues encountered in the earlier practices and has an object of the present invention to provide a stencil printing machine which can prevent stains with printing ink as little as possible in a stationary rest position of each printing drum.
According to a first aspect of the present invention, there is provided a stencil printing machine having a frame body, which comprises a press drum rotatably supported in the frame body, a plurality of printing drums rotatably supported in the frame body in close proximity to the press drum and each including an outer circumferential periphery having a first portion formed with a stencil clamping base, which has a stencil clamping segment and a second remaining portion formed with an ink permeable member, and a plurality of ink supply rollers located inside the printing drums, respectively, for supplying ink to respective inner peripheral surfaces thereof. During printing operation, the plurality of printing drums are rotated with a given rotational phase angle relative to the press drum, and the ink supply rollers are rotated to supply ink to the inner peripheral surfaces of the respective printing drums. At termination of printing operation, the printing drums are kept in respective stationary rest positions wherein the stencil clamping bases of the respective printing drums are out of respective ink stain zones defined by vertical lines intersecting rotation centers of the respective printing drums, horizontal lines intersecting rotation centers of the respective ink supply rollers and the outer circumferential peripheries of the respective printing drums.
According to a second aspect of the present invention, there is provided a stencil printing machine which comprises a frame body, a press drum rotatably supported in the frame body, a plurality of printing drums rotatably supported in the frame body in close proximity to the press drum and each including an outer circumferential periphery having a first portion formed with a stencil clamping base, which has a stencil clamping segment, and a second remaining portion formed with an ink permeable member, a stencil making unit supported in the frame body for making stencil sheets each having a perforated image area formed on the basis of a color original, with the stencil sheets being supplied to and mounted onto the outer circumferential peripheries of the respective printing drums, and a plurality of ink supply rollers located inside the printing drums, respectively, for supplying ink to respective inner peripheral walls thereof. During printing operation, the plurality of printing drums are rotated with a given rotational phase angle relative to the press drum, and the ink supply rollers are rotated to supply ink to the inner peripheral walls of the respective printing drums. At termination of printing operation, the printing drums are kept in respective stationary rest positions wherein the stencil clamping bases of the respective printing drums are out of respective ink stain zones defined by vertical lines intersecting rotation centers of the respective printing drums, horizontal lines intersecting rotation centers of the respective ink supply rollers and the outer circumferential peripheries of the respective printing drums.
According to a third aspect of the present invention, there is provided a method of controlling a stencil printing machine having a press drum rotatably supported in a frame body, a plurality of printing drums rotatably supported in the frame body in close proximity to the press drum and each including an outer circumferential periphery having a first portion formed with a stencil clamping base, which has a stencil clamping segment, and a second remaining portion formed with an ink permeable member, and a plurality of ink supply rollers located inside the printing drums, respectively, for supplying ink to inner peripheral walls of the respective printing drums, and a main motor for driving the press drum and the plurality of printing drums. The method comprises rotating the press drum and the plurality of printing drums with the main motor, making stencil sheets each having a perforated image area formed on the basis of an original, mounting the stencil sheets onto the printing drums, supplying ink to the plurality of printing drums, supplying a print medium between the printing drums and the press drum to allow ink to be transferred from the printing drums through the perforated image areas thereof to the print medium to reproduce a desired image thereon. During printing operation, the plurality of printing drums are rotated by the main motor with a given rotational phase angle relative to the press drum and the ink supply rollers are rotated to supply ink to the inner peripheral walls of the respective printing drums.
At termination of printing operation, the printing drums are kept in respective stationary rest positions wherein the stencil clamping bases of the respective printing drums are out of respective ink stain zones defined by vertical lines intersecting rotation centers of the respective printing drums, horizontal lines intersecting rotation centers of the respective ink supply rollers and the outer circumferential peripheries of the respective printing drums.